The present invention relates to a fabrication method for strained-silicon and, in particular, to compressive strained-silicon.
Chip operating speeds, while desirable, depend on driving current. Improving mobility of the device to increase driving current thereof has become a technique commonly used by chip manufacturers.
In recent years, research has proven that strained-silicon enhances carrier mobility significantly. As shown in FIGS. 1A and 1B, electron mobility increases with tensile strain and hole mobility increases with compressive strain. Electron or hole mobility is higher in MOSFET with a channel of strained-silicon than a conventional MOSFET, of the same size, without strained-silicon. Increased performance is thus accomplished. A current method of fabricating strained-silicon forms the strained-silicon on a relaxed silicon-germanium layer. Since the lattice constant of germanium is 4% larger than that of silicon, the relaxed silicon-germanium layer exerts tensile stress on silicon during formation on the silicon-germanium layer, wherein the silicon-germanium layer is formed on a graded silicon-germanium layer.
Tensile strained-silicon has been realized through several methods such that performance of the NMOS device is enhanced. However, there is no effective method to fabricate the compressive strained-silicon required to improve hole mobility and driving current of a PMOS device, a barrier to application of the strained-silicon technology to integrated circuits.